HF uses radiation-hard quartz fibers as the active detector element. The Cherenkov radition from showering particles passing through the fibers genertes very few photons per GeV of energy incident on the absorber. The photoelectron yield is in fact 0.24 pe/GeV, and the PMTs used as photodetectors deliver about 15-75 fC per photoelectron. The gains vary from 0.06 to 0.3 GeV/fC, with a mean of ~0.1 GeV/fC or 0.15 to 0.9 GeV/linearized QIE count as shown in this distribution. The sensitivity of the HF QIE electronics is nominally 2.7 fC/count, with calibration corrections, more precisely 0.36 ± 0.009 (linearized QIE cnts/fC).
A single photoelectron therefore delivers 6-30 QIE counts.
The rms noise = 0.9 QIE counts = 0.15 - 0.7 GeV. Note that this is well less than the 4 GeV corresponding to a single pe.
The full scale range of the HF QIEs is 27pC = 1.6 - 5.4TeV.
These gains reflect changes made at the end of CRUZET3 (2008) when the HV was increased, decreasing the gains by a factor of 0.4
A complete description of the gains before the these changes can be found in NOTE2006_044